The present invention relates to DC circuit interrupters, and more particularly to DC contactors of the polarized variety.
Contactors for interrupting DC current are frequently used in pairs, connected to each end of a load so that circuit interruption effectively isolates the load from the source of DC power. Although in principle such contactors are simply switches which open and close a circuit in practice they are highly specialized devices, frequently incorporating arc extinguishing mechanisms. Arcs are commonly extinguished in such circuit interrupters by providing an arc chute incorporating a series of fins, plates or baffles, and means for forcing the arc which is drawn between the contacts into the baffle area where it is stretched, cooled, and finally extinguished. An arc, being formed by an electrical current, is responsive to a magnetic field according to the well known right-hand rule, and magnetic fields are commonly utilized to urge an arc from between a pair of separating contacts into an arc chute where it is extinguished or "blown out." The magnetic blowout mechanism may be comprised of an electromagnetic coil, sometimes termed a "blowout coil." More compact, less expensive structures may make use of permanent magnets, in which case a current-carrying coil is unnecessary. With electromagnetic blowout coils, however, load current is used to energize the coil so that the direction of the magnetic field reverses when load current reverses. With both the load (arcing) current and magnetic field reversed, the net effect is the same and the arc continues to move in the same direction, i.e., into the arc chute. However, when permanent magnets are used the magnetic field direction is constant, and hence when the direction of load current changes the magnetic field moves the arc in the opposite direction--away from the arc chute and into the contactor mechanism. Such an occurrence is obviously detrimental to the device, and may destroy it. For this reason it is conventional to establish a mandatory current direction through such devices. Such contactors are referred to as "polarized" contactors. Of course, it is possible to adapt such a device to accept current at either of its terminals by the addition of rectifiers or the like, but in practice such adaptations are uneconomical and conventional practice is to simply mark the terminals with plus and minus signs, to indicate preferred direction of current flow.
In the past this problem has been addressed in numerous ways. For instance, double break individual contactors may be provided, having both polarities of permanent magnets and utilizing the appropriate set of contacts and magnets, depending upon current direction. This obviously requires a redundant set of contacts and magnets, and switching or rectifying apparatus for directing current only through the appropriate contacts. Another, straightforward approach is to control the opening times of two different contactors, so the properly-polarized one is actuated before the other. This requires not only additional sophistication in the control function for selectively timing operation of the contactors, but in addition means for sensing the direction of current flow, and effecting a change in the control system. Alternatively, a line current polarity sensor could be built into a contactor for imparting a delay into the control mechanism by means of timers, RC circuits, or the like. All of these approaches, however, involve the addition of numerous components with the attendent cost and effect upon reliability. It will therefore be appreciated that it would be highly desirable to provide means for causing only the appropriately-polarized one of a set of polarized DC contactors to interrupt load current.
It is therefore an object of the present invention to provide an improved polarized contactor for use with bidirectional DC current.
Another object is to provide a contactor system in which the properly-polarized one of a pair of contactors operates to initially break a circuit.
Yet another object is to provide for the sequential operation of a pair of polarized contactors, in a sequence determined by current direction, which is simpler and more reliable than any heretofore known.
It is another object of the invention to provide a simple, inexpensive means for effecting the sequential operation of a pair of polarized DC relays, depending upon the direction of current flow.